Oil filters intended for use in combustion engines conventionally comprise filter media with fibers obtained from wood pulp. Such wood pulp fibers are typically 1 to 7 millimeters long and 15 to 45 microns in diameter. Natural wood pulp has largely been the preferred raw material for producing filtration media due to its relatively low cost, processability, various mechanical and chemical properties, and durability in the end application.
The filter media are pleated to increase filtration surface area transversally to the direction of the oil flow.
U.S. Pat. No. 3,288,299 discloses a dual type of oil filter cartridge wherein part of the flow is through a surface type of filter element, such as pleated paper, and the rest of the flow is through a depth type of filter element such as a thick fibrous mass. An oil filter and adapter is disclosed in U.S. Pat. No. 3,912,631.
A typical prior art oil filter is shown in FIG. 1. Reference numeral 1 refers to the pleated filter media (or filtration media) and 2 to a backing structure. A conventional filter media exhibits low stiffness and has poor mechanical strength in terms of tensile strength and burst strength. The filter media 1 is therefore used together with a metal mesh or other type of pleat shape when used in the end application.
Nevertheless, in view of the low mechanical strength the filter media tend to burst over time on exposure to engine oil at the temperatures encountered in a combustion engine, such as 125-135° C.
Although filter media products that are produced largely with wood pulp are still an excellent choice for most automotive and heavy duty oil filtration applications, there is a growing market demand for oil filtration products that exhibit increased strength and durability over time as the media is exposed to the various chemical, thermal, and mechanical stresses of the end application environment. This demand stems from both harsher end application conditions that the media is exposed to as well as increasing demand for filter media that can be safely used in the end application for increasingly longer amounts of time without rupturing or failing.
The long-standing and widely applied solution to this demand has been to incorporate some minor quantity of synthetic fiber, typically PET polyester, in the amount of about 5-20%. The result of fortifying the fiber furnish in this way is higher media strength as well as enhanced chemical and mechanical durability when the media is exposed to the end application environment, due to the superior chemical, thermal, and mechanical durability of the synthetic fibers themselves.
For air filters there are alternative technical solutions primarily based on non-natural fibers described in the art.
U.S. Pat. No. 7,608,125 discloses a MERV filter composed of a wet laid fibrous mat comprising about 20-60 wt-% of glass fibers, about 15-60 wt-% of polymer fibers, and about 15-40 wt-% of a binder for bonding of the fibers. The binder of this disclosure is latex modified with melamine formaldehyde.
Published US Patent Application No. 2012/0175298 discloses a HEPA filter comprising a nonwoven web of two different fiber components. The first fiber component is formed by fibers of polyesters, polyamides, polyolefin, polylactide, cellulose esters, polycaprolactone, up at least 20% of the weight of web. The second fiber can be composed of either cellulosic fibres (Lyocell) or glass or combination of the two. There is further a binder component formed by acrylic polymers, styrenic polymers, vinyl polymers polyurethanes, and combinations thereof.
Published US Patent Application No. 2013/0233789 discloses a glass-free non-woven fuel filtration media that is comprised of a blend of a staple synthetic fibers and fibrillated cellulosic fibers.
U.S. Pat. Nos. 7,488,365, 8,236,082 and 8,778,047 disclose further filtration media containing 50 to 100% of synthetic fibers of the weight of the fibrous web.
None of the references to air filters and fuel filter media discloses a filter medium capable of forming a self-supporting oil filter when configured into a pleated structure and which would be capable of working properly at the harsh conditions in connection with a combustion engine.
In fact, the known filtration media containing a high percentage of synthetic fibers are not pleateable or self-supporting as such, and they have to be co-pleated and reinforced with some sort of additional mechanical support layer, such as a plastic or wire mesh backing. Media made with high levels of synthetic fiber typically tend to exhibit drape and they lack sufficient stiffness and rigidity causing the pleats to collapse without an additional support. A 100% synthetic media as disclosed in the art cannot maintain a grooving pattern like corrugation or a pleated structure due to the thermal and mechanical properties of the synthetic fibers.